A rotation angle measuring apparatus, also called a rotary encoder, is a type of apparatus used to measure angles and rotation speeds, and is widely used in many fields at present, e.g. can be used to measure electric machine speed or for position control systems, etc. According to the principle by which detection is carried out, rotary encoders can be divided into optical encoders, magnetic encoders and capacitive encoders, etc. Capacitive rotary encoders are more precise and more robust than optical and magnetic encoders.
A capacitive rotary encoder in the prior art comprises a moving disk (also called a rotor) and a stationary disk (also called a stator). The moving disk can rotate about an axis (e.g. a rotation axis of a rotating component such as an electric machine), so as to generate a rotation angle relative to the stationary disk. The moving disk comprises a reflecting region, while the stationary disk comprises an emitting region and a receiving region; a static electric field can be formed between the emitting region and the receiving region on the stationary disk, and the static electric field so formed is modulated by the reflecting region on the moving disk as the moving disk rotates. Thus, based on an output of the receiving region on the stationary disk, the rotation angle of the moving disk can be determined. The reflecting region on the moving disk generally has a periodic shape, e.g. a shape such as a repeating sine, cosine or triangular wave. As the moving disk rotates, the static electric field formed between the emitting region and the receiving region on the stationary disk is modulated by the reflecting region with the periodic shape on the moving disk, and correspondingly develops a periodic variation, with each repeating shape of the reflecting region representing one period of the static electric field. By determining the angle through which the moving disk rotates in one period of the static electric field, the incremental rotation angle of the moving disk (i.e. the rotation angle of the moving disk relative to each period starting point) can be determined. Furthermore, by determining the initial position of the moving disk, the absolute rotation angle of the moving disk can be determined (i.e. the actual rotation angle of the moving disk relative to a zero point).
In different application fields, it is sometimes necessary to determine the absolute rotation angle of the moving disk in one circle, and sometimes it is necessary to be able to determine a multiple-circle rotation angle of the moving disk.